Gas dynamics in the star forming region G18.148$-$0.283: Is it a manifestation of two colliding molecular clouds?

Abstract

We report the results obtained from a multi-wavelength study of the HII region, G18.148$-$0.283, using the upgraded Giant Metre-wave Radio Telescope (uGMRT) at 1350 MHz along with other archival data. In addition to the radio continuum emission, we have detected the H169$\alpha$ and H170$\alpha$ radio recombination lines towards G18.148$-$0.283 using a correlator bandwidth of 100 MHz. The moment-1 map of the ionized gas reveals a velocity gradient of approximately 10 km s$^{-1}$ across the radio continuum peaks. The $^{12}$CO ($J$=3$-$2) molecular line data from the COHRS survey also shows the presence of two velocity components that are very close to the velocities detected in the ionized gas. The spectrum and position-velocity diagram from CO emission reveal molecular gas at an intermediate velocity range bridging the velocity components. We see mid-infrared absorption and far-infrared emission establishing the presence of a filamentary infrared dark cloud, the extent of which includes the targeted HII region. The magnetic field inferred from dust polarization is perpendicular to the filament within the HII region. We have also identified two O9 stars and 30 young stellar objects towards the target using data from the 2MASS, UKIDSS, and GLIMPSE surveys. Cumulatively, this suggests that the region is the site of a cloud-cloud collision that has triggered massive star formation and subsequent formation of an HII region.